That was not fun.

pangeo_forge_runner/bakery/flink.py

@@ -388,12 +388,7 @@ def bake(self, pipeline: Pipeline, meta: ExecutionMetadata) -> None:
     @classmethod
     def validate_bake_command(cls, bake_command: HasTraits) -> List[TraitError]:
         errors = []
-        print("HERE, LOOK HERE.======================")
-        print(bake_command._trait_values)
-        print("====================================")
-        print(bake_command.container_image)
-        print("END, LOOK HERE.======================")
-        if not bake_command.container_image:
+        if not bake_command._trait_values["container_image"]:
             errors.append(
                 TraitError(
                     "'container_image' is required when using the 'FlinkOperatorBakery' "

pangeo_forge_runner/commands/bake.py

@@ -47,6 +47,21 @@ class Bake(BaseCommand):
         """,
     )
 
+    bakery_class = Type(
+        default_value=LocalDirectBakery,
+        klass=Bakery,
+        config=True,
+        help="""
+        The Bakery to bake this recipe in.
+
+        The Bakery (and its configuration) determine which Apache Beam
+        Runner is used, and how options for it are specified.
+        Defaults to LocalDirectBakery, which bakes the recipe using Apache
+        Beam's "DirectRunner". It doesn't use Docker or the cloud, and runs
+        everything locally. Useful only for testing!
+        """,
+    )
+
     recipe_id = Unicode(
         None,
         allow_none=True,
@@ -88,23 +103,6 @@ class Bake(BaseCommand):
         """,
     )
 
-    # Note: This needs to be last, it appears, to ensure that other fields are set first.
-    # TODO: Terrible design. Rethink using traitlets if we can't defer to post-initialization.
-    bakery_class = Type(
-        default_value=LocalDirectBakery,
-        klass=Bakery,
-        config=True,
-        help="""
-        The Bakery to bake this recipe in.
-
-        The Bakery (and its configuration) determine which Apache Beam
-        Runner is used, and how options for it are specified.
-        Defaults to LocalDirectBakery, which bakes the recipe using Apache
-        Beam's "DirectRunner". It doesn't use Docker or the cloud, and runs
-        everything locally. Useful only for testing!
-        """,
-    )
-
     @validate("job_name")
     def _validate_job_name(self, proposal):
         """
@@ -123,8 +121,7 @@ def _validate_job_name(self, proposal):
             )
         return proposal.value
 
-    @validate("bakery_class")
-    def _bakery_impl_validation(self, proposal):
+    def bakery_impl_validation(self):
         """
         Validates the 'bakery_class' trait using class-specific validation logic.
 
@@ -135,9 +132,12 @@ def _bakery_impl_validation(self, proposal):
         Raises:
         - TraitError: If multiple validation errors are encountered, a single TraitError
         is raised containing a summary of all error messages.
+
+        Note: Traitlets has no convenient way to set initialization hooks (???) so we have
+        to run this manually.
         """
-        klass = proposal.value
-        if errors := klass.validate_bake_command(proposal.owner):
+        klass = self.bakery_class
+        if errors := klass.validate_bake_command(self):
             if len(errors) == 1:
                 raise errors[0]
             error_messages = "\n".join([str(error) for error in errors])
@@ -187,6 +187,9 @@ def start(self):
         """
         Start the baking process
         """
+        # Validate bakery-specific requirements of this class. Yes, we have to do it here.
+        self.bakery_impl_validation()
+
         if not "pangeo-forge-recipes" in [d.metadata["Name"] for d in distributions()]:
             raise ValueError(
                 "To use the `bake` command, `pangeo-forge-recipes` must be installed."

tests/unit/test_bake.py

@@ -79,6 +79,7 @@ def test_job_name_validation(job_name, raises):
             ),
         ):
             bake.job_name = job_name
+            bake.bakery_impl_validation()
     else:
         bake.job_name = job_name
         assert bake.job_name == job_name
@@ -100,6 +101,7 @@ def test_container_image_validation(container_image, raises):
         ):
             bake.bakery_class = "pangeo_forge_runner.bakery.flink.FlinkOperatorBakery"
             bake.container_image = container_image
+            bake.bakery_impl_validation()
     else:
         bake.bakery_class = "pangeo_forge_runner.bakery.flink.FlinkOperatorBakery"
         bake.container_image = container_image